A measuring technical problem in electroimpedance tomography is that the useful signal used to calculate the graphic representation must be sufficiently larger than the particular interferences. The simple increase in the measuring current has limits, because the currents that are permissible according to the standards are limited (in a frequency-dependent manner). Consequently, it is necessary to reduce the interference signals. Moreover, the interference signals consist partly of self-generated interferences, e.g., the crosstalk or the so-called common-mode signal, which increase proportionally to the increase in the current. Increasing the measuring current can improve the distance from the external interferences at best.
Electrical impedance tomography (EIT) is a method for reconstituting impedance distributions or, in case of functional EIT for reconstituting impedance changes relative to a reference distribution, in electrically conductive bodies. A plurality of electrodes are arranged for this purpose on the conductive surface of the body being examined, and the control unit, usually a digital signal processor, ensures that a pair of (preferably) adjacent electrodes each is supplied consecutively with an electric alternating current (for example, 5 mA at 50 kHz), and the electric voltages are detected at the remaining electrodes acting as measuring electrodes and are sent to the control unit. The impedance distribution or, in case of functional electroimpedance tomography, the change in that impedance distribution relative to a reference distribution can be reconstructed with suitable algorithms by the combination of the measured voltage values during the consecutive rotating current feeds. A ring-shaped, equidistant arrangement of 16 electrodes is used in typical cases, and these electrodes can be placed around the body of a patient, for example, with a belt. Alternating current is fed into two adjacent electrodes each, and the voltages are measured between the remaining currentless electrode pairs acting as measuring electrodes and recorded by the control unit. By rotating the current feed points, a plurality of measured voltage values are obtained, from which a two-dimensional tomogram of the impedance distribution can be reconstructed relative to a reference in the plane of the electrode.
Such tomograms are of interest in medicine because the impedances depend on the biological state of the organs (for example, the breathing state of the lungs) and/or the frequency of the current. Therefore, both measurements at different states are performed at a given feed frequency and in different biological states (for example, observation of the breathing cycles) and measurements at different frequencies performed at different feed frequencies and identical biological state in order to obtain information on the corresponding impedance changes. As was already mentioned, functional impedance tomography of the lungs, in which the electrodes of the EIT device are arranged around the patient's thorax, is an important application. One of the interferences occurring in terms of measuring technique during impedance tomography is the ultimately unavoidably occurring residual asymmetry of the alternating current feed on the body, which also occurs when a symmetrical AC power source is used, which is due to the differences in the routing of the cables to the different electrodes, different contact resistances, etc.
The power source supplies an alternating current alternating between 20 kHz and several MHZ for the measurement. To evaluate the causes of the development of the asymmetry of current feed, it is consequently necessary to use not only disturbing differences in the ohmic resistances but also those in the AC impedances. The use of alternating current is necessary for medical reasons. The permissible measuring currents would be even lower by several orders of magnitude in case of direct current. Moreover, the measurement with alternating current makes possible a low-drift, frequency-selective demodulation of the measuring currents and to obtain information on how the impedances of the upper body change with the frequency.
FIG. 3 shows a basic circuit diagram of an electroimpedance tomograph of the type mentioned in the introduction, which embodies a symmetrical AC power source due to the insertion of a power source 22 or an isolation transformer 40 between the AC power source 22 and the selector switch (multiplexer) 60. The primary circuit of this isolation transformer 40 has clear references and consequently usually asymmetries to the ground (the measuring technical reference point of the device) due to the circuitry. To keep the effect of the asymmetry on the secondary side as limited as possible via stray capacitances, a shield winding, which is grounded, is located between the two windings. If no asymmetrical stray capacitances of the secondary side are desired against this shield winding, the secondary winding must have a symmetrical design in relation to the shield winding. This symmetrical design has, of course, limits, so that the stray capacitances must be assumed to be different on both sides of the secondary winding in relation to the ground in an equivalent circuit. This is only one example of how extensive symmetry of the alternating current feed is sought to be achieved.
FIG. 4 shows an equivalent circuit to explain the asymmetries of the AC signal applied in the device from FIG. 3. Asymmetries in the power source are only part of the asymmetries occurring in the measuring circuit. Other causes are the multiplexer or selector switch 60, which has different conducting-state DC resistances RML and RMR for the two terminals (depending on the channel being used) and also different stray capacitances CML and CMR in relation to the electric environment. The multiplexer 60 is followed by the shielded connecting line, so that the capacitive differences in CLR and CLL against the ground of the two connecting lines to the electrodes are to be taken into account. The inductive and resistive line impedances ZLL and ZLR are other sources of asymmetry especially in case of differences in the lengths of the connecting lines and at high measuring current frequencies.
Finally, the transition impedances of the electrodes against the skin surface are finite and different, which is likewise to be taken into account. Moreover, they are complex, i.e., they are composed mainly of the transition resistances REL and RER and the transition capacitances CEL and CER.
All asymmetries combined cause that there are different flows of measuring currents from the two lines via the stray capacitances against the ground and different voltage drops at the longitudinal impedances and consequently there are differences in current flow between the two feed terminals, because more or less different current components will have now flown to the ground before and the differential current flows to the ground via the body resistance and the transition impedance of the reference ground electrode and thus it generates a common-mode signal on the body and consequently on the measuring electrodes. This common-mode signal is different for all actuated electrode positions both because of the differences in the channels of the multiplexer 60 as well as the external lines and of the electrode transition resistances and generates at the measuring amplifier error signals, which may overlap the useful signals, together with the value and the differences of the transition impedances of the particular measuring electrodes (which are connected by the multiplexer 60) with the finite common-mode reduction resulting therefrom.
Even if the measuring amplifier behind the multiplexer were ideal, the electrodes of the particular connected measuring lines would again generate asymmetries and only a finite common-mode signal suppression in a manner that is the reverse of what happens in case of the current path via the parasitic impedances and the values thereof, which differ from one measuring channel to the next.
One possibility of keeping this common-mode signal as low as possible is a reference ground electrode with a very low transition impedance. The size of the possible reference ground electrodes and their ability to be handled are limited and, beginning from a certain size, they generate movement artifacts, which originate from the changes in the transition impedance that are generated during the movement of the patient. Therefore, this measure only has limited effectiveness.